Run of Mine (ROM) material is defined as any free material from a mine, including both ore and waste material. In the economic analysis of the viability of a mine, a vital consideration lies in the assessment of the cost of transporting (ROM) material following extraction.
Typically, the material is removed by purpose built trucks, capable of transporting many tens of tonnes of material at a time. The ROM material is loaded onto the trucks, either from temporary stockpiles or directly from extraction. The trucks then travel from the pit to the waste material dump or to the ore crushing plant, depending on the quality of material for the particular load.
The purchase of a fleet of appropriate trucks involves a considerable capital cost, as does the establishment and maintenance of roads of sufficient bearing capacity to bear the load of heavily laden trucks. Further, as the mining process progresses, the extraction point moves deeper, the path to be followed by the trucks lengthens. For a typical road grade of 10%, this means for every meter of depth, an extra 10 meters of road is required. Consequently, the cycle time for the trucks to retrieve material, dump it and return, increases. It follows, therefore, that there will be an ever increasing unit cost of ROM material over time, as the mine progresses. This variable cost, combined with the substantial amortized capital cost, have an enormous detrimental effect on the viability of mining operations.
As an alternative method, it is known to adopt conveyor systems to remove this material. Conventional conveyor systems are characterized by reinforced rubber belts, supported beneath the belt by idlers and some form of drive system to motivate the belt. The system is in a fixed position, and must be substantially straight, having little or no ability to deviate either horizontally or vertically.
A conveyor system needs to be tensioned between drive drums, which provide the motivating drive, thus being a substantial contributor to the lack of deviation. Therefore, it is normal, for such systems, to spend a considerable amount of time selecting the path to ensure an uninterrupted straight path is achievable.
A further problem lies with the mine environment. As would be expected, there is a considerable volume of dust, mud and water, which infiltrate and generally envelope all equipment within the mine zone. The combination of these factors provide for a natural and effective lubricant. Once in contact with the belt, the drive capability of the drive drums is markedly reduced. Whilst this can be accommodated for in a horizontal orientation, when the belt is inclined, the traction force is reduced below a useful level, and drive can be substantially lost.
It also follows that, when inclined above a certain angle, the material will slide down the belt, preventing transport. Typically, such systems are restricted to about 20° from the horizontal.
A further problem is the size limitation of the system. The belt of the conveyor system is susceptible to severe damage from large, angular rocks, partly because of the punching shear force established between the sharpness of the rocks on the belt and the support from underneath, as a result of the impact of the rocks. As a consequence, such systems are associated with In-Pit Crushing plants, to reduce the size and weight of individual rocks to be transported. Typically such systems are limited to rock sizes of less than 300 mm, and thus any material to be removed from the pit must be crushed to a suitable size.
A significant problem associated with this arrangement is the cost of crushing. The establishment and operation of a crushing plant is significant, and is an operation which is, justifiably, only performed if absolutely necessary. Unfortunately, it has been shown in practice that, typically, ROM material comprises a very high percentage of material above 300 mm, up to a common maximum of 1000 mm, not withstanding extremely large “renegade” rocks of unpredictable size. Thus, this initial crushing is necessary, but based on the limitations of the conveying system rather than on a sound economic basis.
Further, because material must first be delivered to the crushing plant, normally be truck, then loaded to some type of conveyor system, the material is having to be “double handled”. With every transfer of material between transport mediums representing additional cost per tonne processed, further detracting from the economic viability of such a system.
Further still, whilst the ratio of waste material to ore can vary substantially, economic forecasts for a mine's viability often rely on a ratio of 6:1, that is for every 1 tonne of ore there is 6 tonnes of waste. Using the truck based system, this waste material is taken directly to a dumping site, as crushing of such material is of no practical benefit, and represents an enormous and pointless waste of resources. However, for a conveyor system, crushing of waste material above 300 mm is essential, so that it can be transported out of the pit.
An alternative form of conveyor system is the so-called suspended belt system, as exemplified by U.S. Pat. No. 4,915,213. This system, marketed under the name SICON, includes a belt that is tear-drop shaped, and open at the top. The longitudinal edges of the belt, adjacent to each other at the top of the tear-drop shape, are mounted on continuous cables tensioned between end drums that drive the belt through friction. Material is loaded by separating the cables, allowing the belt to open. After loading, the cables are brought together, containing the material not unlike a sack. It has been found that such an arrangement cannot be used on a significant slope without slippage of material thereon.
Other similar known conveyor systems are disclosed in Australian patent application AU 55345/94 (WO 95/11848 and also as RU 2118284) and U.S. Pat. No. 5,083,658, which disclose hose or continuous (endless) conveyor systems for bulk materials. Each of these systems in particular discloses the longitudinal edges of the endless belt having thickened force absorbing edges used to support and drive the endless belt.
AU 55345/94 in particular further discloses that wear of the edges can be compensated by the ‘V’ edges lying deeper in the space bounded by the drive rollers, and that this compensation avoids the need to move the driving rollers closer together.
The system shown in U.S. Pat. No. 5,083,658 discusses continuous longitudinal edge force absorbing members each with a continuous central rope core. The system is intended to allow the belt to readily traverse corners. This specification takes no account of wear or damage occurring to the longitudinal edge force absorbing members or to the belt.
Another known system is disclosed in Soviet Union patent publication SU 1795952, wherein the endless belt is supported at edges thereof by bolt on support brackets. If the endless belt becomes worn or damaged, the brackets could in theory be removed and reapplied to a replacement endless belt, though the ensuing down time and loss of production would make this process inefficient and high cost. It would be more usual to replace the entire belt and brackets or cut out a section of belt and weld in a fresh section. Removing a worn or damaged section of an endless belt also requires lengthy downtime with corresponding loss and disturbance to production, with corresponding increase in costs and time.
Typically, worn or damaged endless belts are repaired by cutting out the worn or damaged section, and bonding, welding and/or bolting a replacement section into place. With endless belts utilizing continuous thickened or strengthened longitudinal edge portions, such as in U.S. Pat. No. 4,915,213 or AU 55345/94, and also those systems where such longitudinal edge portions incorporate continuous rope cores, such as in U.S. Pat. No. 5,083,658, removing a section of belt also requires the longitudinal edges of that section to be removed. Joining the replacement section to the original belt and edges can introduce inherent weaknesses into the belt system, not least because the edges are used to support the belt and load. Also, and more importantly, the edge portions take the tensile loads applied in moving the belt, and therefore require strong continuous edges. Replacement bonded or spliced joints can introduce weaknesses, with potential failure of the joint under tensile load or problems in traversing pulleys.
Another problem with known continuous haulage systems is a that each system is initially designed and constructed for a particular application or site, and they generally lack adaptability to be readily extended or change direction as the site e.g. long wall mine site, extends its workings, or for disassembly of the system for removal, such as to another site. One known system provides a conveyor belt joined by a mechanical piano type connector. Such connectors have to endure large tensile and shear forces because they bear part of the downward weight of the load, as well as tensile longitudinal forces in the direction of conveying, thereby often resulting in premature wear and failure.
Other types of belt are formed as endless belts without a specific mechanical connector. The need to extend the belt when extending the conveyor system, or failure of such belts, is usually rectified by splicing in a new section of belt to avoid the need to provide a completely new longer or repaired belt. There is a lengthy and complex procedure in rejoining the new section to the previous ends, resulting in lengthy downtime and loss of productivity.
When such connections or belts fail, it is often necessary to replace the mechanical piano type connector completely or splice in a new portion of belt.
furthermore, it is difficult to form a strong secure connection between an end of the original edge portion and an end of the new edge portion such that the joint can reliably take longitudinal forces in conveying the belt and load. This is especially the cased where the edges have a rope core, such that the rubber/polymeric edge material has to be stripped back to expose a bare section of rope core in both the old and new sections, and the two rope ends then spliced together, the rubber/polymeric edge material then needing to be replaced or bonded back over the spliced rope join.
This arrangement leads to a stiffer join which can cause problems when the belt requires flexibility to traverse horizontal or vertical return wheels, corners or pulleys. Inflexibility or long joins potentially leads to failure or more repairs. Also, where a repair is possible, downtime in replacing the damaged or worn section and joining edge portions is both lengthy and costly.
Other known art is disclosed in Australian patent application AU 200112556 (WO 01/3603) by the present applicant, the contents of which are incorporated herein by reference.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of admission that the prior art forms part of the common general knowledge in Australia.